Natural brines that exist in salt deposits such as the Atacama Salt Deposit contain considerable amounts of sodium, potassium, boron, lithium, and calcium in the form of chlorides and sulfates, as well as other elements in smaller quantities such as bromine, strontium, silica, iron, iodine, etc.
Owing to the commercial value of some of these elements or compounds that can be obtained from brines, the brines are generally processed by solar evaporation in order to crystallize out the desired product or products or to eliminate others of less commercial value. In the first stages of crystallization, brines that are high in chlorides produce salts such as halite (NaCl), silvite (KCl), silvinite (NaCl, KCl), carnallite (KCl, MgCl2.6H2O), and bischofite (MgCl2.6H2O). Brines with sulfates can precipitate salts such as epsomite (MgSO4.7H2O), kieserite (MgSO4.H2O), gypsum (CaSO4.2H2O), singinite (CaSO4, K2SO4.H2O), schoenite (Na2SO4, K2SO4.6H2O), and lithium sulfate (Li2SO4.H2O) , in addition to other double salts of sulfates and chlorides such as kainite (KCl, MgSO4.2.75H2O) and lithium schoenite (LiSO4 K2SO4).
When the goal is to recover the lithium in the form of hydrated lithium sulfate (Li2SO4.H2O) contained in brines that are high in sulfates, such as those from Atacama Deposit, the lithium sulfate is contaminated with other sulfates that can precipitate with it, such as kainite, epsomite, schoenite, or lithium schoenite, even though other salts can also precipitate at various stages of the crystallization process, such as chloride salts if they are present in sufficiently high proportions, e.g., bischofite and carnallite. All other salts are undesirable if the intention is to recover lithium since they contaminate the product and it is difficult and expensive to purify the product if the goal is to obtain lithium sulfate of high purity, above 98-99%.
In order to produce lithium sulfate from natural brines from salt deposits, various techniques have been proposed such as precipitating double compounds of lithium as a final product. For example, in Chilean Patent No. 33,867 by N. Parada and L. F. Vergara (xe2x80x9cProcess for Producing Potassium Sulfate and Lithium Salts from a Mixture of Double Salts of Lithium, Potassium, and Magnesiumxe2x80x9d), sodium, potassium, and magnesium salts are precipitated in succession until a mixture is produced of composite salts of schoenite and lithium schoenite, which are then separated by selective flotation followed by lixiviation with water at room temperature in order to obtain a solid with a lithium schoenite purity of approximately 95%. This brine is then reacted with potassium chloride at room temperature by means of a metathesis reaction in order finally to obtain a lithium chloride brine that can be used to precipitate lithium carbonate by conventional means by precipitating it with soda ash.
In another Chilean patent, No. 31.513 from the Comitxc3xa9 de Sales Mixtas de Corfo [Corfo Committee for Mixed Salts] (xe2x80x9cNew Process for Extracting the Lithium Contained in the Salt Deposits of the Atacama Salt Deposit in the Form of Crystallized Monohydrated Lithium Sulfatexe2x80x9d), brines that are first concentrated by solar evaporation and have lithium contents of 11-14 gpl, such as lithium chloride, are treated with brines that are high in magnesium sulfate in an interchange reaction or xe2x80x9csalting-outxe2x80x9d that makes it possible to precipitate monohydrated lithium sulfate. The resulting solution is again evaporated until 11-14 gpl of lithium is obtained, and the process is repeated in order to separate additional hydrated lithium sulfate. The process, however, requires considerable quantities of magnesium sulfate (generally in the form of epsomite, MgSO4.7H2O) that has to be added at approximately 50% excess above stoichiometric in order to implement the displacement reaction:
2LiCl+MgSO4xe2x86x92Li2SO4.H2O+MgCl2.
However, the product that is obtained (hydrated lithium sulfate) has a purity of only 95%, with 5% precipitated magnesium sulfate; this limits the subsequent use of the lithium sulfate that is obtained in this way.
In another patent that relates to the precipitation of lithium sulfate from brines (U.S. Pat. No. 4,287,163 by D. E. Garrett and M. Laborde xe2x80x9cProcess for Recovering Lithium Monohydratexe2x80x9d), a process is described that is similar to that set forth in Chilean Patent 31.513 (since the Comitxc3xa9 de Sales Mixtas de Corfo is a participant in both) in which a soluble sulfate, such as magnesium sulfate or sodium sulfate or even sulfuric acid, is added to brines from the Atacama Salt Deposit that are first evaporated in order to concentrate them and that have lithium contents of more than 0.4% by weight and less than 30 moles of magnesium chloride per 1000 moles of water, in order to precipitate out the hydrated lithium sulfate, whereby the salt that is preferred for this process is epsomite (MgSO4.7H2O), which is precipitated by cooling the brine. The process produces a low-purity lithium sulfate since when, for example, magnesium sulfate is used as the salt for precipitating lithium, the lithium sulfate that is obtained has a purity of 95.5%, with 4.3% precipitated magnesium sulfate and a lithium recovery rate from the brine of only 32%. When sodium sulfate is used, the precipitated lithium sulfate is also of low quality, with a content of 24.32% of lithium sulfate, which is contaminated with 18.5% sodium sulfate and 52.1% sodium chloride and a lithium recovery rate of only 6.3%. If the precipitation agent is replaced with sulfuric acid, the product that is obtained is also of low quality since it has 22.8% lithium sulfate contaminated with 77.2% boric acid and lithium recovery rate from the brine 17.6%.
U.S. Pat. No. 4,723,962 by V. C. Mehta (xe2x80x9cProcess for Recovering Lithium from Salt Brinesxe2x80x9d) describes another process in which monohydrated lithium sulfate is precipitated from natural brines that contain chlorides and sulfates through an initial step of cooling said brine to a temperature of between 4 and 10xc2x0 C. in order to precipitate NaCl, followed by the addition of water and subsequent cooling of the resulting brine to 0xc2x0 C. in order to precipitate hydrated magnesium sulfate, then evaporating the resulting brine to approximately 90% of the saturation value of lithium, whereby it is then mixed with starting brine in proportions such as to obtain a potassium-lithium molar ratio of less than approximately 0.35. The brine mixture that is obtained in this way is cooled to 0xc2x0 C. in order to precipitate carnallite (MgCl2 KCl.6H2O) , and then the evaporation of the resulting brines continues, but salts of this brine are kept from precipitating. Then magnesium sulfate (epsomite) is added in order to produce a sulfate level of 60% by weight compared to the total water contained, under which conditions the monohydrated lithium sulfate precipitates. After various stages of washing, filtration, and drying of the crystals, the above-described process produces a hydrated lithium sulfide with a purity of 92% and a lithium recovery rate of 76% from the lithium-concentrated brine.
As will be noted, in all of these patents the processes use a salt (sulfate), generally of magnesium, such as epsomite in order to precipitate the hydrated lithium sulfate, either by a simple interchange reaction (salting-out), by cooling the resulting brine in order to precipitate it, or by precipitation as double lithium compounds and then lixiviation, whereby in all cases a monohydrated lithium sulfate is obtained with a purity of no greater than 95% and variable recovery rates that cannot exceed 76%.